12TH INTERNATIONAL

BRICK/BLOCK Masonry c O N F E R E N C E

Gab

REHABllITATION OF THE CHAPEl, BOWDOIN COllEGE, BRUNSWICK, MAINE, USA

B. Gabbi and K. Cash 2

'Gabby, Brent, Staff Engineer, Simpson Gumperu &; Heger Inc.,

297 Broadway, Arlington, MA 02474-5310, USA

'Cash, Kevin B., Principal, Simpson Gumpem &; Heger Inc.,

297 Broadway, Arlington, MA 02474-5310, USA

ABSTRACT

In the outumn of 7996, Simpson Gumpertz ond Heger Inc. (SGH) wos retoined to in­vestigote the severe bulging ond crocking of the north bell tower of the Chopel at Bowdoin College in Brunswick, Moine, USA. The granite Chopel, which was designed by New York architect Richord Upjohn in 7844, is considered to be one of the most important symbols on the Bowdoin campus. The Germon Romanesque building hos a typical ecclesiasticalloyout with o westfront flanked by two slender 36.5 m (720 ft) bell towers (north ond south) with stone spires. SGH conducted an investigotion of the north bell tower to understond the distress ond to recommend repairs for the building. This paper describes the investigotion of the tower's condition using both destructive and non-destructive techniques; a finite ele­ment onalysis used to expio in vertical crocking in the tower; mock-ups of potential re­pairs; ond final repair options considered.

Key words: Stone, tower, non-destructive, testing, onolysis, repair, investigotion

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1. BACKGROUND AND DESCRIPTION OF THE CHAPEl

The Chapel at Bowdoin College in Brunswick, ME is both an important symbol to the identity of the Bowdoin College tradition and life, and an excellent example of mid-19th century construction practices using stone masonry. The German Ro­manesque chapel, designed by Richard Upjohn in 1844, is symmetrical in plan w ith two 36.5 m (120 ft) high bell towers along its westfront, a central nave, a transept, and two side aisles.

The two predominate faces of the bell towers (north and west, and south and west for the north and south towers, respectively) have centrally located, equally spaced loophole windows throughout the height of each elevation. Above the 10-ophole windows are larger louvered openings for the belfry, which are on ali four sides. Each tower has a steep granite spire, which extends approximately 10.5 m (35 ft) above the belfry arches (Photo 1). There is a single bell in the north tower, and a multi pie bell carillon in the south tower.

The footprint of each tower is roughly 3,700 mm by 3,700 mm (12 ft by 12 ft). The tower walls are approximately 915 mm (3 ft) thick and consist of a 200 to 300 mm (8 to 12 in.) exterior granite wythe laid in a random ashlar pattern, a 300 to 400 mm (12 to 16 in .) granite rubble core, and a 300 mm (12 in.) inte­rior wythe constructed of flat granite fieldstone. At random locations, the exterior wythe of stone is laid as a header to tie the exterior wythe to the interior core. At loophole windows, the interior wythe and part of the core is replaced by splayed

Photo 1.

brick arches that were once covered with plaster. Intermediate wood landings were once framed into the exterior walls below the windows, but most have be­en removed, either for new access ladders or due to deterioration. The belfry flo­ors are also wood framed and are covered with multi pie layers of roofing mate­riais.

In the autumn of 1996, Simpson, Gumpertz & Heger Inc. was retained to inves­tigate the severe bulging and cracking of the north bell tower, to make a cursory investigation of the south bell tower, and to recommend repairs. Because the south bell tower is relatively stable compared to the north tower, this paper will concentrate on the north tower only. This paper will describe the investigation process, repair techniques attempted to stabilize the masonry, and final repair so­lutions considered.

2. FIElD INVESTIGATlON

Before the causes of the distress in the north tower could be diagnosed, we first documented the existing conditions, including the extent of cracks and the bulging. Our work started with a topographic survey of each exterior fa­cade to map the severity and location of the bulging and other distress. With the aid of a laser levei, we established a rotating vertical infrared beam of light parallel to each elevation, and marked x and y coordinates on the masonry sur­faces (approximately 600 mm [24 in .] vertically and 400 to 600 mm [16 to 24 in.] horizontally). We measured the distance from the vertical plane of light to the masonry at each coordinate point to establish a "negative" profile of the facade. Access to each point on the tower was gained using a crane and bas­ket. As part of this work, we recorded the location of major cracks in the ex­terior facades, and the relative displacement of each of the tower corners. Our field information was placed in a spreadsheet and imported to mapping soft­ware to create a "positive" topographic map. The map then was superimpo­sed over an electronically scanned image of the tower. Our crack data was ad­ded manually.

During our initial investigation, we also reviewed the condition of the tower inte­rior. Water stains and heavy efflorescence typically emanate from around the pe­rimeter of the belfry floor (the roof has obvious splits and open areas), and at the head and perimeter of the loophole windows. The interior plaster coating around the windows is loose, crumbling, or missing. In general, however, the interior sto­ne wythe is plumb and in good condition with solid joints.

Bulges and Vertical Cracks

Ali four elevations of the north tower contain bulges in the exterior masonry wyt­he. To eliminate erroneous readings due to the rough faced nature of the grani­te, we defined "bulges" as locations, which are more than 40 mm (1-1/2 in.) out-

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of-plane of the surrounding masonry. The most severe bulges typically occur bet­ween the top of the fou rth floor window and the bottom of the belfry on each elevation . Additionally, minor bulges are also adjacent to the loophole windows. Our survey reveals that the bulges on the north tower vary from 48 mm (1 -7/8 in .) to 140 mm (5-1/2 in.) out-of-plane.

To better understand the extent and underlying conditions at the bulges, we re­moved small stones next to several bulges. The removed stones were chosen ca­refully so as not to destabilize the bulges. At each probe location we noted a void between the back of the exterior wythe and the core. The bottom of each void was filled with a semi-consolidated wet sand. The size of the void correlated with the size and amount of the wal l bulge. Each probe location also revealed a solid core that appeared in a sound condition.

We also documented the locations of the vertical cracks both on the exterior and interior of the walls. Vertical cracks are prevalent throughout the entire height of the tower, and ali the corners are loosing their interlock. The north and west ele­vations of the north tower have vertical cracks that propagate to the interior walls . These "through" cracks occur in the upper third of the wall between the heads (arches) and sills of windows.

3. DIAGNOSIS OF MASONRY DISTRESS

Bowing

We concluded that the bowing is due to a cyclic failure mechanism that is rela­ted to exposure. Bulging tends to increase with increasing height, and is particu­larly associated with the belfry floor. Water penetration through the exterior wyt­he, around windows, and principal ly at the belfry floor starts the deterioration processo Water absorption in the outer portion of the core and subsequent free­zing creates a cyclic cracking of the cement binder that is washed away by sub­sequent water penetration . As the areas of deterioration increase in size, ice len­ses form that exert horizontal force on the exterior wythe, forcing it out-of-plane. The sand tends to concentrate at the base of the created void, where it can ab­sorb more water, re-freeze and expando This process is consistent with our ob­servations of less dist ress in the south tower, where the belfry floor remains wa­tertight and water staining and damage is minimum.

Vertical Cracking

We surmised that horizontal thrust from the stone spire was the primary factor to the vertical cracking . As a way to prove our hypothesis, we created a two-dimen­sional finite element model of the tower to determine the state of stress in the masonry. Our analysis assumed that the base of the wall is not restrained and the Poisson ratio of the masonry to be 0.15.

Based on the self weight of masonry tower and spire, we calculated the stresses to be on the order of 200 to 275 kPa (30 to 40 psi) in the horizontal direction. We felt that this range of horizontal tensile stress was high enough to initiate ver­tical splitting in masonry with deteriorated mortar.

To further understand this phenomenon, we created a secondary model with ver­tical splits above and below the belfry. Horizontal tensile stresses of the same magnitude occur under the sill of the top loophole window and further up the gable of the tower. These stresses indicate that the tower was "unzipping" due to its own weight. We concluded that the levei of "unzipping" would continue to occur and would increase as the mortar deteriorated further.

To arrest further movement until repairs could be made, we recommended that the tower's perimeter be banded. The banding, which consisted of 100 mm (4 in.) wide, 3 mm (1/8 in.) thick stainless steel strapping, was mounted on the out­board face of the tower and secured to the masonry with through bolts and an­chor plates. Five bands were installed around the upper third of the tower where the bowing and cracking is the most severe.

4. REPAIR METHODOlOGY

Before any comprehensive repair work was recommended, we conducted an ex­tensive literature search and consulted other design professionals throughout the United States. We were also guided by the Secretary of the Interior's Standards for the Treatment of Historic Properties and the Guidelines for Preserving, Reha­bilitating, Restoring and Reconstructing Historic Buildings (SIS). This document, as well as other international conservation charters, played a significant role in the decision making processo

We identified two potential repair schemes based on two of the philosophical ap­proaches defined in SIS. The first approach would follow that of Preservation and repair the tower in-situo SIS defines Preservation as the least disruptive of ali the philosophical approaches and emphasizes protection, maintenance, limited re­pairs, and mini mal replacement. As an added advantage this approach would be least expensive.

The second, a more radical approach, would follow that of Rehabilitation, and re­move and re-build (with the existing masonry) the exterior wythe. Rehabilitation as defined by SIS stresses the protection of character defining features. However, when the existing fabric is heavily damaged or deteriorated, more repair and re­placement can be considered.

After our literature search and discussions, we decided, in concert with the Colle­ge, to pursue the Preservation approach and further investigate the potential for in-situ repairs. To be effective, in-situ repairs had to fulfill three basic require­ments:

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• Confine the existing masonry both in-plane and out-of-plane to prevent furt­her movement.

• Fill voids and consolidate the loose sections of the core to reduce water pene­tration and stiffen the exterior wythe.

• Re-establish the composite nature of the wall.

To confine the masonry, we proposed installing in-plane and out-of-plane stain­less steel anchors. The out-of-plane anchors would be installed at regular intervals throughout the height of the tower and would mechan ically t ie the exterior and interior wythes together. The in-plane anchors would be installed within the thickness of the walls between the loophole windows on each elevation. These anchors would provide tensile resistance against further vertical cracking in the field of the masonry.

To fill the voids and stiffen the exterior wythe, we proposed injecting the tower w ith a low compression, elastic cementious grout. The grout would be formula­ted to closely match the properties of the existing core. As part of this work, the three most significant bulges would have to be re-built to return the stone to a reasonable plane.

Once the above work was complete, we would perform a "deep pointing" of the exterior wythe. This would serve to both redistribute stresses and to improve the water tightness of the restored wall .

Before any comprehensive repa ir work was implemented, we planned a mock-up of the repair concepts to check the feasibil ity and efficacy of the repair techni­ques. The mock-up work included grout injecting a relatively small area of the to­wer, and install ing one in-plane anchor. Out-of-plane anchors are simpler to ins­tall than the in-plane anchor, and are done often on many build ings. As such, they were not included in the mock-up.

As part of the mock-up work, we also proposed using direct pulse transmission techniques as a potential method of assessing the performance of the grouting process, and to quantifying the levei of core consolidation. Acoustic measure­ments were recommended before and after the grouting process to monitor the increase in the masonry's density. Also, we explored the use of th is technique as a quality contro l measure during more extensive repair work.

5. IN-SITU REPAIR MOCK-UP

The first step of the mock-up was to define the mock-up location and the extent of grouting that would be necessary. The mock-up area was located in the lower 4.6 m (15 ft) of the north tower and was limited to the north and west elevations. Both the elevations were cracked, bulged, and out-of-plumb, except for an area

south of the first loophole window on the west elevation. We conducted a bo­roscope survey in the proposed mock-up area to understand the levei of separa­tion between wythes and competency of the interior masonry. We found areas of voids between the core and the exterior wythe, some areas of consolidated sand in the hollow sections, and a solid core.

As part of this work, we also removed severa I 50 mm (2 in.) diameter cores from the walls. The samples were sent to Atkinson &. Noland &. Associates (ANA) of Boulder, CO. for the testing and grout formulation . The physical testing and grout formulation served two purposes:

• To verify the compatibility of physical properties (compressive strength and mo­dulus of elasticity) between the proposed grouts and existing building materiais.

• To verify the ability of grout mixtures to fill voids and bond to existing building materiais .

Based on the boroscope survey and the laboratory testing, ANA recommended that two different grouts be formulated. The first, a coarse grout, would be used for voids greater than a 6 mm (1/4 in.) deep and a second, fine grout, for those less than a 6 mm (1/4 in.) The constituents of the coarse grout included a find sand aggregate and portland cement as the main binder, with a relatively high fly ash and lime content to provide mix stability, without compromising harde­ned properties. The constituents for the fine grout included of a fine cement slurry consisting of Type 111 cement, fly as h, and lime. No aggregate was added to the fine grout. Both grouts contained a shrinkage compensator and superplas­ticizer to limit the water demando

In parallel with the ANA work, we conducted our own petrographic analysis of the mortar. Much to our surprise, a course grain portland cement was found in the original mortar. The cement probably was imported from Europe, since the manufacture of portland cement did not occur in the United States until the 1870's (cement was first used in Great Britain in the 1820's). The addition of ce­ment in the original mortar gave us added assurance that a highly elastic cement based grout would not damage the tower and would be compatible with the ori­ginai materiais.

As part of the quality control of the grout, ANA performed a series of tests to de­termine grout flow and stability properties. The tests on the grout included wa­ter retention, expansion and bleeding, and flow characteristics. ANA conducted further testing to determine compression and shear bond strengths of the ma­sonry and grout. The average compressive strength and modulus of elasticity of the grout was found to be 14 MP and 2,070 MP (2,010 psi and 300,000 psi) res­pectively. Both the low compressive strength and stiffness were desirable proper­ties to allow for flexibility in the grouted matrix. The grout also bonded well to granite test samples, which gave us further confidence that the composite natu­re of the wall could be re-established .

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5.1. Field Mock-up

After the laboratory testing work was complete, we returned to the field with ANA to implement the field mock-up on the north tower. Grout was injected between the exterior wythe and core, and its success was measured by direct-pulse trans­mission techniques and by sampling selected areas of the wall after the grout had cured. As part of the mock-up, we also installed an in-plane anchor (by Cintec­Harke) below the sill of the first loophole window.

Masonry Solutions Inc. of Baltimore, MD injected the masonry' and ANA supplied engineering quality control for the grout mix and conducted the acoustic eva­luation. General Concrete Cutting and Coring of Ottawa, Ontario cored the ma­sonry and installed the in-plane anchor.

Grout Injection Process

Before the grouting process began, severa I attempts were made to install supple­mental self-tapping stainless steel masonry ties throughout the mock-up area. These ties were meant to help resist the lateral pressure of the fluid grout. Due to thinness of the mortar joints between adjacent stone, the ties could not be insta­Iled without getting "hung-up" on the granite. Instead, it was decided to limit the height of the grout lifts to reduce the hydrostatic head of the fluid grout.

The grout was introduced into the wall via pre-drilled injection ports in the mor­tar joints. The grout was injected under low pressure (14 kPa [2 to 3 psi]) into 10-wer ports, until it came out of higher port locations. This was an indication that the void or crack had been filled. The grouting process occurred throughout the mock-up area until ali the ports had been filled with mortar.

Pulse velocity measurements were taken by initiating an energy source on one si­de of the wall (in this case a calibrated hammer blow), and measuring the rate of travei of the induced shock wave through the masonry. The denser the material, the faster the wave travei time.

ANA compared the pre- and post-injection pulse measurements of the mock-up area, which showed a dramatic increase in velocities. The increase in velocities in­dicated a significant gain in overall wall density resulting from the grout injection. A comparison of the contour maps clearly indicates a marked increase in overall wall density. ANA also created a histogram of pre- and post- through wall velocities to quantify the contour maps. The histogram indicates that approximately 50% of the pre-injection velocities were at or below 500 m/s. Conversely, the histogram of the post-injection velocities indicates that approximately 5% of the velocities are bet­ween 1500 and 2000 m/s with the majority being between 2000 and 3500 m/s.

As an added quality assurance measure, and to confirm the post-injection pulse velocity measurements, cores were removed from the mock-up area to determi-

ne the levei of consolidation. The mock-up area was heated for a month while the grout cured, prior to the coring.

In-Plane Anchor Installation

After the grout injection process was complete, the in-plane anchor was installed. The hole for the in-plane anchor was formed with a dry-coring technique to avoid pumping large quantities of water into the masonry, which is normal with tradi­tional wet-coring methods. On the north elevation, the stone located below the first-floor loophole window and approximately 18 in. in-board from the west fa­ce of the tower was removed. A 2 in. diameter hole was drilled into the heart of the wall and extended to the in-board face of the exterior wythe of the south ele­vation. A continuous 10 ft-6 in. stainless steel Cintec anchor was placed in the completed hole and expanded with cement grout. The exterior stone removed from the north face was then replaced, making the installation of the anchor in­visible.

5.2. Post-Injection Evaluation

To evaluate the curing and bonding of the grout to the granite and rubble cen­ter, six cores within the mock-up area were removed from the tower. After the core removal, little to no grout was contained within the cores. In an attempt to understand the reasons for the perceived lack of grout behind the bulged areas, several stones were removed to ex pose the heart of the wall.

A total of four stones from the exterior wythe were removed to expose the core of the wall. The removed stone revealed a significant quantity of sand (mortar that had lost its binder) in the space between it and the core. Some grout was found in the sandy interior, but most was located in the void areas adjacent to the stone.

The sand around each of the stones was removed in an effort to find and trace grout paths. We found that grout filled the gross voids and the "tunnels" that lin­ked some of the larger voids together. However, the quantity of sand in the co­lIar joint greatly outweighed the quantity of grout injected.

Although the mortar contained in the exterior wythe and collar joint was highly deteriorated, we found that the competency of the masonry was clearly demar­cated. Once ali the sand between the exterior wythe and core was removed, we found a very hard, competent mortar and granite core.

As stated earlier, an area south of the first loophole window on the west elevation of the north tower is in relatively good condition compared to other areas of ma­sonry in the mock-up. To understand the differences between this apparently go­od masonry and the bowed areas, two stones were removed. We found that the-

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re was some sand between the exterior wythe and the core, but the quantity was significantly less than in the bowed areas we probed.

The post-mock-up cores and probes demonstrate that there are some limitations with the acoustic evaluation techniques in deteriorated rubble core masonry. Even though the velocity profiles show an increase in wall density, they fail to identify an increase in wall consolidation. These techniques alone cannot be used as the final determinate for wall consolidation. Post-injection cores must be taken to measure the efficacy of these techniques, especially in old, deteriorated rubble core masonry.

6. Al TERNATE REPAIR OPTIONS

Before abandoning in-situ repai rs entirely due to the lack of consolidation, we dis­cussed alternate repair options with severa I architectural conservators. Based on our conversations and professional judgment, we re-evaluated this approach to include the instal lation of more anchors, which would help to confine the sand and help re-establish the composite nature of the wall.

6.1. In-situ Repairs with More Anchors

This option is based on the concept of confining the sand between the exterior wythe and the core and not allowing it to move further. If properly confined, sand will transfer axial load, but is limited in its ability to transfer shear stresses. To achieve confinement and re-establish the composite nature of the wall, seve­ra l hundred anchors would have to be installed throughout the stonewall. The an­choring could be done either from the inside or the outside. Anchoring from the outside would be more reliable, but would leave holes that require repair. Pat­ching the granite to an acceptable visual appearance would be difficult. Mortar patches have different wetting characteristics that would be obvious in the rain. Even if stone can be found to match, installing "dutchmen" over the anchor ho­les wou ld be difficu lt and produce a spotted appearance.

Anchoring f rom the inside would alleviate the visual problem, but there would be a risk that the drilling may push the exterior wythe off the building, and the an­chors would not praperly engage the exterior wythe due to the prof ile of the back of the stone.

Regardless of the anchor placement method, this option would require graut in­jection to fill the gross voids between the exterior wythe and the core. If the gross voids were not filled, there would be a risk of further deterioration as sand settles into these spaces with time. The voids wou ld also serve as pockets where ice len­ses may form, which would push out the exterior wythe and permit greater wa­ter entry into the structu re. The inability of the grout to consolidate the large quantities of sand sti ll would leave avenues for long-term water migrat ion within

the walls, which would decrease the effectiveness and longevity of the grout in­jection.

Along with grout injection, severa I of the bulges also would have to be rebuilt. Reconstruction of the bulges would be required before the grout injection work could begin beca use the fluid grout could de-stabilize the bulged masonry.

With this option pointing (100 to 150 mm [4 to 6 in .] in depth) the exterior wyt­he also would be required to help re-.distribute the load paths, and reduce water penetration. Leaving the sand in place would complicate the pointing process be­cause the pointing mortar would not have a competent material in the joint to which to bond. This could lead to a premature failure of the pointing mortar.

This option is not a large departure from the initial proposed grouting repair. Ho­wever, this option requires many more anchors than first anticipated. Its chief downfalls are the potentially poor (spotted) final appearance, and increased maintenance costs (re-pointing and general overall deterioration due to water mi­gration in the masonry) with time. Based on these concerns, it was decided to re­move and re-build the exterior wythe of masonry.

7. FINAL REPAIR RECOMMENDATIONS AND CONCLUSIONS

The removal and reconstruction of the exterior wythe of masonry is most the de­pendable repair option and has fewer unknowns with respect to the structural adequacy and longevity of the repair. On the surface, this option may seem mo­re intrusive than the in-situ repair. However, the large quantity of repair anchors required for the in-situ repair would mean that more historic fabric (holes drilled in both exterior and interior wythe) would be removed in the process of the re­pairo With the reconstruction option, more material would be removed tempora­rily, but its removal would not involve the permanent loss of historic fabriCo

Before any material would be removed, each stone on the tower would be num­bered and its location accurately documented photographically with elevation measurements. Undocumented remova I of the exterior wythe would require the masonry be replaced in a conjecturai manner. This type of repair would lead to greater loss of historic fabric, due to the necessity of cutting stone to create a new random ashlar pattern.

As part of the reconstruction process, anchors would be added to attach the ex­terior wythe to the back-up, and a soft, compatible cement-based grout would be used to fill the collar joint between the stone and the remaining core. Howe­ver, the number and size of the anchors would be significantly less than of the in­situ repair and would be located in bed joints (not through individual stone). Be­cause water penetration in the masonry is a significant contributor to the overall deterioration, flashings at the tops of window arches and below windowsills also would be installed.

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Along with removal and reconstruction of the exterior wythe, we also recom­mended that the belfry floor be removed and replace in-kind. This would require that the bell be removed before the work begins. As part of this work, we re­commended that new flat-seam copper cover the floor, and the belfry louvers be installed at an increased pitch and closer spacing to reduce the quantity of wind­driven rain into the belfry area.

During the entire reconstruction phase, we concluded that it would be possible for the spire to remain in place. Temporary supports would be designed to resist the dead weight of the spire while work would be conducted on the masonry be­low. Some spire work would be required such as repointing, and removal and re­installation of internai cross braces.

CONCLUSIONS

In summary, this project confirms the need for through evaluation and study of ali issues before decisions are made regarding repair options. Moreover, it de­monstrates the need for mock-ups to establish the efficacy of repairs, especially on historically sensitive structures.